I don't know what wrong with me. I'm sixteen, I'm usually happy and animated, but lately I have been slugish depressed and I have mood swings were I snap at everyone and quickly im back to just not caring. I have gained 20 lbs in a month and I can't fall

Seek therapy. You appear to be reporting symptoms of depression. But it would best for you to seek a therapist for an assessment and therapy.
Diet. It may be your diet. Get rid of sugar. Processed foods. And grains and legumes. Paleo diet.
Depression . Depression could explain you mood changes, weight gain, sleep difficulty and headaches. Research shows that people who engage in therapy and medication get better faster and stay better for a longer period of time than either therapy or medication alone. You should call your doctor and discuss your symptoms with them. They may be able to help you, or at least refer you to someone in your area who can help. I added a link to a website with helpful information about depression. There are a few other illnesses that can cause similar symptoms such as hypothyroidism (low thyroid hormone) and your doctor may choose to check for this before starting medication. Good luck!
Depression/Endocrine. Weight gain, mood swings, loss of energy, irritability, and loss of motivation are symptoms of depression. Depression can be caused by a number of things (ex. environment, diet, lack of sleep, medication/ drugs, thyroid, hormones, medical conditions). Consider an endocrine work up as well as speaking with a psychologist.